Discovery of laser has brought great changes to human civilization. In 1960, Maiman succeeded in producing the first ruby laser in the world. Since the laser has characteristics such as high energy density, high directivity and coherence, it has a wide range of applications in many areas and has been combined with a number of disciplines to form several application technologies, such as photovoltaic technology, laser medical treatment and photobiology, laser processing, laser detection and measurement technology, laser holography, laser radar, laser guidance, laser isotope separation, laser controlled nuclear fusion, laser weapons, and so on. The laser obtained directly by using a laser crystal is limited in wave band, while using the frequency conversion crystals can convert a laser at a limited wavelength into a laser in a new band, which is a key means of obtaining new laser sources. However, the key issue of achieving high-efficiency conversion in laser wavelength is the availability of a high-quality and excellent-performance non-linear optical crystal.
In 1961, Franken et al. observed the frequency-doubled effect in the quartz crystal for the first time, which not only marked the naissance of the non-linear optical discipline, but also strongly promoted the development of non-linear optical material science. Non-linear optical crystal has frequency conversion effect, electro-optic effect, photorefractive effect etc., and can be used in the fields of laser frequency conversion, electro-optical modulation, signal processing and the like. It is an important basic material for the growing optoelectronics industry and an important mainstay for the development of solid-state laser technology, infrared technology, optical communication and information processing and other fields, and is playing a more and more important role in scientific research, industries, transportation, national defense, health care and the like. Over the past decade, although the non-linear optical crystals for blue/green light and ultraviolet (UV) and deep ultraviolet (DUV) have attracted broad attention, these crystals have their limitations and have been difficult to obtain a wider range of applications. For example, the β-BaB2O4 (BBO) crystal has photorefractive effect at its quadruplicated frequency, which hinders the application of quadniplicated frequency. Additionally, the β-BaB2O4 (BBO) crystal has a small receiving angle and a large walk-off angle, so in order to obtain high conversion efficiency, it is required that the pump laser beam has good beam quality (low divergence angle and good mode). The BBO crystal would be deliquescent to some extent, causing some inconvenience while using. The LiB3O5 (LBO) crystal is deliquescent to some extent as well. Neither of these two crystals can generate a light below 200 nm by direct frequency doubling. The layers of the KBe2BO3F2 (KBBF) crystal are associated with each other via ionic bonds between K and F, i.e. the bonding force between the layers is weak, for this reason, it has a strong layer-by-layer growth habit and is difficult to grow thick, difficult to process and easy to dissociate, and thus difficult for practical use. Therefore, there is an urgent need to develop a practical new non-linear optical crystal having better performance and matching shorter wave band, and many countries around the world are actively looking for such a new non-linear optical crystal.
In recent years, when developing new non-linear optical crystals, it is not only focused on the optical and mechanical properties, but also more emphasizing on the preparation characteristics. It is hoped that the new crystal materials are easy to prepare so as to obtain low-price, high-quality and large-size non-linear optical crystals. Under the guide of the design concept of introducing alkaline earth metal cations into the boron-oxygen framework to improve the performance, the anions, taking boron-oxygen primitives as the foundation, have large band gaps, small probability of two-photon absorption and high laser damage threshold, and is helpful to obtain strong non-linear optical effects; the B—O bonds facilitate transmission of broad band light. The cations selecting from alkaline earth metal ions have no transition of d0 electrons in the ultraviolet region, which is conductive to ultraviolet transmission.